In manufacturing production lines, it often is desirable to have a paint system capable of painting successive workpieces, such as automobile bodies moving on a conveyor, different colors as they are conveyed past a spray station. As a consequence, both manual and automatic systems have been developed for changing color as successive workpieces are painted. In many applications, it also is desirable to use an electrostatic coating applicator which imparts a high voltage charge to the paint as it is atomized. Electrostatic spray painting has many advantages including producing a more uniform coating on irregular surfaces and reducing the amount of paint needed to coat a workpiece through an increased coating material transfer efficiency.
In recent years, there has been an increased desire to use water based paints whenever possible. Using water as the paint solvent is less expensive than other solvents and water is not harmful to the environment. Many problems have occurred in attempting to combine a color change system with an electrostatic coating system when an electrically conductive paint is used. When a water based paint or other electrically conductive liquid is applied with an electrostatic system, it is necessary either to totally isolate the paint supply from electrical ground or to provide a voltage block between the paint supply and the coating applicator. For a system of the first type capable of applying different color paints, supplies for all paints must be isolated from ground. Typically, all of the paint supplies will be at a high voltage during painting and color change.
Such a system has several disadvantages. A system of this type may be dangerous to personnel working in the area. A very large mass will be charged to a high voltage. This high electrical load is often sufficient to prevent the high voltage power supply from maintaining a desired voltage at the spray gun. The high electrical capacitance of the charged mass will result in a dangerously high quantity of electrical energy being stored in the system. Also, where the system permits all of the paint to be charged from the spray gun back to their source, it is not possible to perform maintenance work on any portion of the system while the spray gun is in operation. For example, while the system is painting workpieces with red paint, it is not possible to fill a different color tank, such as a green paint tank, with additional paint.
In a typical system using a voltage block for isolation, the voltage block is achieved by dripping individual droplets of the paint into a reservoir which is isolated from ground and supplying paint from the isolated reservoir to the coating applicator. Due to the conductivity of the paint, the reservoir will be at the same high voltage as the applicator. The individual droplets of paint break the circuit continuity between the grounded supply tanks and the reservoir. This arrangement is not easily cleaned for sequentially applying different color paint and is not suitable for rapid color change. In order to decrease the time required for color change, some systems provide a separate isolated reservoir for each color paint, as illustrated in U.S. Pat. No. 4,085,892, for example. Each of these reservoirs remains at the high voltage during painting and color change.
In a voltage block system shown in U.S. Pat. No. 4,232,055, different color electrically conductive paints are supplied from tanks which are individually isolated from ground and from each other. Each tank is located in a separate grounded cage. The tanks are connected through insulated hoses to a color change manifold. Only the tank currently supplying paint through the manifold to the electrostatic applicator will be charged to a high voltage due to the conductivity of the paint. Maintenance may be performed on any of the other paint tanks which are individually grounded when the cage enclosing each tank is opened. When painting with a selected color is completed, paint is purged from the insulated supply hose and the hose is dried to form a voltage block. This system still requires charging a relatively large mass to the same voltage to which the atomized paint is charged. Also, color change is delayed by the time required to purge and dry relatively long paint supply hoses connected to the supply tanks.